uclibc: disable posix_madvise for noMMU

Now that the madvise defines are disabled for noMMU targets,
architectures with MMU and noMMU support as ARM are failing to compile
in noMMU mode, since the defines are used internally in posix_madvise.c.

Disable compilation for posix_madvise() for noMMU.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>